The present invention relates to a positioning control apparatus for use in a driving device of a machining apparatus using a servomotor or the like, and more particularly to a positioning control apparatus comprising a motor, a digital type position detector, a digital type speed detector and a feedback circuit, in which the operation is carried out in accordance with the speed of an input pulse and the amount of the input pulse.
In the prior art, positioning control systems such as shown in FIG. 1 or FIG. 2 have been employed for controlling positioning devices of the type described above. The difference between the control systems of FIGS. 1 and 2 is in the feedback circuit. That is, although the signal from a tacho-generator 3 as a speed detecting device is directly used as a speed feedback signal in the control system of FIG. 2, a feedback pulse from an encoder 2, which is a digital type position detector, is employed as a speed feedback signal D after being converted into a voltage signal by a F/V converter 8. As will be understood from the above description, these control systems are essentially of the same type.
Therefore, the operation of the conventional control system will be described in the case of the system of FIG. 1. When an input pulse C is applied to the system, a comparing means in the form ofa counter 4 which is used as a detecting means for detecting the positional difference between a target position of the object and an actual position thereof counts the input pulses C to increase the content of the counter 4. As a result, the output level of a D/A converter 5 is also increased and a control signal in the form of a driving instruction signal A is applied to a motor 1 through a subtracter 6 and a driving amplifier 7. The operation of the positioning device or the operation of the motor 1 at this time is detected by means of the encoder 2 and the resulting feedback pulses B from the encoder 2 are applied to the counter 4 to decrease the content of the counter 4 in accordance with the feedback pulses B. The feedback pulses B are also applied to the F/V converter 8 to convert the feedback pulses B into a speed feedback signal D. The speed feedback signal D is applied to the subtracter 6 in which the subtraction operation between the signals D and A is carried out to form a feedback loop.
In such a prior art positioning control system, as shown in FIG. 3, the relationship between the content of the counter 4 (abcissa) and the signal A (ordinate) which is provided to the driving amplifier 7 becomes linear. In general, since a frictional load necessarily exists in a positioning driving mechanism, there is a neutral zone in which the object is not able to move unless a torque more than the frictional load is provided to the motor. This zone is indicated by hatching in FIG. 3.
Therefore, in the case that the object is moved one increment when one pulse is applied to the motor, there is the disadvantage that the object starts to move only when the content of the counter 4 exceeds a predetermined value. In addition, since it is impossible to carry out the moving operation in response to each one pulse, the accuracy of the positioning is low. Furthermore, since the magnitude of the frictional load in the positioning driving mechanism depends upon the location of the object, an irregularity of operation will occur when the pulse speed is not so high. These problems may be solved if a large torque motor is employed and the torque of the motor exceeds over the value of the neutral zone even when the content of the counter 4 is increased by a value corresponding to one input pulse. However, the use of such an extremely large torque motor is disadvantageous because the controlling system becomes large. In addition, due to the frequent starting, stopping and reversing operations of the positioning mechanism, much power consumption is required due to the large inertial force of the large torque motor. Therefore, it becomes difficult to minimize the apparatus and to save energy.